1. Technical Field
This application relates to a backlight unit, more particularly, a backlight unit including a light emitting diode (LED) and a feedback circuit, and a method of driving the backlight unit.
2. Discussion of the Related Art
Flat panel display (FPD) devices that have a relatively light weight, a thin profile, and low power consumption characteristics are being developed and commonly used as a substitute for cathode ray tube (CRT) devices. Generally, display devices may be classified according to their ability for self-emission, and may include emissive display devices and non-emissive display devices. Emissive display devices display images by taking advantage of their ability to self-emit light, while the non-emissive display devices require a light source since they do not emit light by themselves. For example, plasma display panel (PDP) devices, field emission display (FED) devices, and electroluminescent display (ELD) devices are commonly used as emissive display devices. Liquid crystal display (LCD) devices may be categorized as non-emissive display devices commonly used in notebook and desktop computers because of their high resolution, capability of displaying colored images, and high quality image display.
An LCD module of the LCD devices includes an LCD panel for displaying images to an exterior and a backlight unit for supplying light to the LCD panel. The LCD panel includes two substrates facing and spaced apart from each other, and a liquid crystal material interposed therebetween. Liquid crystal molecules of the liquid crystal material have a dielectric constant and refractive index anisotropic characteristics due to their long, thin shape. In addition, two electric field generating electrodes are formed on the two substrates, respectively. Accordingly, an orientation alignment of the liquid crystal molecules can be controlled by supplying a voltage to the two electrodes, wherein transmittance of the LCD panel is changed according to polarization properties of the liquid crystal material. However, since the LCD panel is a non-emissive-type display device, an additional light source is required. Thus, a backlight unit is disposed under the LCD panel, wherein the LCD device displays images using light produced by the backlight unit. In general, backlight units may be classified into two types according to a disposition of the light source, such as side-type backlight unit and direct-type backlight unit. As display areas of the LCD devices become larger, direct-type backlight units including a plurality of light sources have become more commonly used in order to provide increased brightness.
Generally, discharge lamps, such as a cold cathode fluorescent lamps (CCFL) or external electrode fluorescent lamps (EEFL), are used as a light source of the backlight unit. Additionally, a light emitting diode (LED) may be used as the light source of the backlight unit.
FIG. 1 is an assembly view of a backlight unit including an LED of an LCD device according to the related art. In FIG. 1, an LCD device 1 includes a LCD panel 10 and a backlight unit 20 under the LCD panel 10. Here, the backlight unit 20 includes a plurality of printed circuit boards (PCB)s 22 arranged with a stripe type and a plurality of LEDs 24 in each PCB 22 where the LEDs 24 are arranged in series.
For example, the LEDs 24 displays white light by color-mixing through simultaneously lighting red, green and blue LEDs, which emit red, green and blue color light, respectively. LEDs 24 may be repeatedly arranged on the PCB 22 by a unit referred to as an array 26 which may include two to ten LEDs 24. The array 26 may be driven by a drive circuit.
FIG. 2 is a schematic circuit diagram of a backlight unit according to the related art. In FIG. 2, a backlight unit 1 includes an array unit 35 that includes an input terminal 32, an output terminal 34 and an array 30 including an LED 38. Further, the backlight unit 1 includes a switching unit 40 and a control unit 50 between the input terminal 32 and the array 30. The control unit 50 controls an operation of switching unit 40. The array 30 consists of a plurality of LEDs 38 that are connected to each other in series. A driving voltage “Vin” of array 30 is input at input terminal 32 and an output voltage “Vout” is output to the output terminal 34. The driving voltage “Vin” is applied to array 30 through the switching unit 40, such as a field effect transistor (FET). Control unit 50 controls the switching unit 40 through an external signal.
LEDs 38 have an inverse brightness change in accordance with their temperature. As the emission time of LED 38 increases, the temperature of LED 38 also increases. This increase in temperature reduces the brightness of LED 38.
FIG. 3 is a graphic of a brightness relationship between a junction temperature and brightness of an LED. As the temperature of an LED is increased, its brightness may be reduced. For example, when the temperature of a green LED is more than approximately 80 degrees Celsius (° C.), its brightness is reduced to approximately 80 percent (%). Further, when the temperature an LED is more than approximately 120 degrees Celsius (° C.), the LED may not be driven any more.
The longer the backlight unit is active the greater the temperature of the LEDs. The increased temperature leads to an increased reduction in the LEDs' output brightness, which results in image quality degradation. Therefore, there is a need for a backlight unit that senses and compensates for the degradation of LED brightness.